SETTLING LABORATORY
OBJECTIVE
The purpose of this experiment is to determine the average settling velocity of particles in storm
water run-off necessary to achieve 80 % removal in the storm water retention basin.
BACKGROUND
The objectives of storm water retention basins are to reduce the likelihood of flooding as a
result of storm events and to remove particulate matter and other contaminants. The majority of
contaminants in parking lot run-off will be in form of particulate matter. If given sufficient
residence time in the retention basin particulates may settle out from the run-off before it is
discharged. Due to the hydraulic conditions in conventional retention ponds and the particulate
concentration in the run-off Type I settling may represent a reasonable approximation of the
sedimentation process occurring in retention ponds.
For Type I settling the settling velocity will depend on particle diameter and particle density.
Since particles from storm water are inherently heterogeneous the average settling velocity of the
particles can only be determined with the knowledge of their size distribution. Alternatively the
basin design could be based on an observed settling behavior of a representative sample.
EXPERIMENTAL PROCEDURE
In this experiment the necessary settling velocity of particles in run-off will be determined to
achieve 80 % removal in the retention basin. The water has been collected over the past rain
falls at Cheel Parking and analyzed for its solids content. A surrogate mixture of the parking lot
run-off has been created. The synthetic mixture has similar turbidity than the real run-off water
and its particles have similar settling characteristics.
Equipment:
The equipment to be used for this experiment consists of:
1. stop watch, timer
2. turbidity meter or spectrophotometer
3. volumetric and other glassware as needed
Procedure:
1. Take three 500 mL beakers and number them 1-3.
2. Transfer 500 mL of surrogate water to each jar.
3. Mix each jar gently and take one sample to determine the water’s initial turbidity.
4. Place beakers on a solid surface and let solids settle. Try not to disturb the settling process
through moving the beaker.
5. Take 20 mL of sample at precisely 3 cm below the water level (submerge the pipet to the
mark at the pipet) at 2 min, 5 min, 12 min and 25 min of settling. Analyze each sample for
turbidity removal.
DATA ANALYSIS AND RESULTS
Due 9/27/00 before class.
The laboratory write-up should contain the following points:
1. Summarize the experiment briefly.
2. Prepare a table of the raw turbidity data using a spreadsheet program such as Excel.
3. Using your spreadsheet determine the standard deviation for of the turbidity for each of the
four time measurements (standard deviations between the three samples taken at the same
time).
4. A graph of the turbidity as a function of sample time. Plot the average turbidity between the
three jars and the error bars for each time point. Describe the trend of the data.
5. A graph of turbidity removal as a function of average settling velocity for each measurement
time point. Discuss the shape of this graph. Is it a linear curve or does the slope of the
curve change? What implications does a change in slope have on the design of the retention
basin? (Hint: try to plot the graph with changing scales (log. vs. liner) of x and y- axis to
determine the best representation of a model).
6. Based on your results of your experiment which settling velocity would achieve at least 80
% turbidity removal at a depth of 3 cm below the water table?